(1) Field of the Invention
The invention relates to a system for the transmission of n-level data signals from a transmitter to a receiver, the transmitter comprising a carrier oscillator and a modulator connected thereto for generating an angle-modulated carrier having a substantially constant amplitude and a continuous phase.
Numerous modulation methods for an efficient data transmission over telephone lines have been developed and introduced. Substantially all these modulation methods require a modulated carrier signal having amplitude variations, and make use of linear modulators and amplifiers.
However, these modulation methods are not so suitable for data transmission over radio links, because in radio communication systems a high power efficiency requires the use of components having a non-linear amplitude transfer function and the spectrum at the output of such a component, for example a class-C amplifier, will be wider than at the input if the amplitude of the signal at the input varies. Therefore, radio communication systems preferably utilize modulation methods which require a modulated carrier signal of a substantially constant amplitude (envelope), which means the use of angle modulation.
The every growing need for systems for data transmission over radio links also imposes the requirement on the modulation methods to be used therefor of an efficient use of the bandwidth of the available transmission channel, a low level of the power outside the band of this transmission channel also being necessary in view of the large differences in the signal levels of adjacent transmission channels. Even if there is used for that purpose a modulation method which requires an angle-modulated carrier signal having a continuous phase, the spectrum of this carrier signal will nevertheless nearly always be wider than that of the equivalent base band signal. Limiting this spectrum by means of a channel filter is an unattractive technique for radio communication systems, as the practical realisation of such a filter with an accurately prescribed amplitude and phase characteristic and, usually a very small relative bandwidth in the radio frequency range is particularly difficult and, in addition, many systems are of the multichannel type in which the transmitted carrier frequency must be capable of assuming a great number of different values. Therefore, it is necessary to effect in radio communication systems a possible limitation of the spectrum of the angle-modulated carrier signal by means of premodulation techniques.
A further requirement for the modulation methods to be used in radio communication systems is that the receiver must also be capable of a reliable detection of the data signals, whether or not unknown frequency shifts between transmitter and receiver occur. In addition, the detection methods used in the receiver must result in an error probability as a function of the signal-to-noise ratio which degrades as little as possible with respect to the error probability for an optimum baseband transmission of the data signals. To satisfy this requirement to the highest possible extent it must be possible to use coherent demodulation in the receiver and--in view of the required efficient use of power and bandwidth--the carrier and clock signal references required in the receiver must be capable of being recovered from the transmitted modulated carrier signal itself.
(2) Description of the Prior Art
For the transmission of data signals large-scale use is made at present of a modulation method which is known as PSK (Phase-Shift-Keying) and which is described in detail in chapter 10 of Reference D(1) and in chapter 9 of Reference D(2). If the PSK-method is used in its basic form, it results in a modulated carrier signal of a constant amplitude and the data signals can be detected in an optimum manner by means of orthogonal coherent demodulation, it being possible to recover the carrier and clock signal references required therefor from the transmitted PSK signal itself.
Thus, this PSK-method has many properties which are desirable for efficient data transmission over radio links. The power density spectrum of the PSK signal is, however, relatively wide and the level of the spectral side lobes decreases only slowly for an increasing frequency spacing from the carrier frequency, so that interferences having an impermissibly high level can be produced in adjacent radio transmission channels.
The techniques for limiting this PSK spectrum, which have already been used for a long time for efficient data transmission over telephone lines (see, for example, reference D(3)) are unattractive for radio communication systems, because they result in a modulated carrier signal having amplitude variations and, in addition, use channel filters in many cases. In view of the many desirable properties of the PSK method, much attention has been paid these last few years to premodulation techniques for reducing the spectral side lobes without sacrificing the desired properties. In these premodulation techniques the pulse shape for the modulating data symbols is chosen so that the modulated constant-amplitude carrier signal in the centre of each symbol interval has the same nominal phase as when the PSK method is used, and the transitions between consecutive nominal phases are made as gradual as possible. Reference D(4) comprises a good survey of pulse shapes already proposed and the reduction of spectral side lobes achieved therewith. Reference D(5) describes a method for determining the error probability as a function of the signal-to-noise ratio in these cases.